When I first arrived at NASA in 1997 as a student, it was all about Mars. The human spaceflight division buzzed with excitement; there was a sense that the agency might really be about to embark on a new chapter of exploration—the next small step.
There was a kind of Mars underground at NASA, a cadre of folk who had long held dear the hope of sending a human crew to the Red Planet. For them, low Earth orbit and the Moon were pedestrian destinations. Mars was where the action was at; exploring it would be the defining feat of their generation: a long overdue return to the sort of barefaced ambition that had first made NASA famous. A badge had appeared on the lapels of the faithful: a cheap tin badge about the size of a quarter with the words MARS OR BUST! in bold red lettering.
We’ve imagined sending people to Mars since well before Gagarin’s first spaceflight. Wernher von Braun, principal architect of the Saturn V launcher that delivered Neil Armstrong and Buzz Aldrin to the Moon, laid out his dreams in the 1953 publication Das Marsprojekt (The Mars Project), the first mature study of what it would take to send humans across the huge void of space that lay between Earth and Mars.
It was a design of startling ambition. In it von Braun envisaged an armada of ten spacecraft plowing on toward their destination, crewed by no less than seventy astronauts. In this plan, he foresaw the need to place nearly forty thousand tons of payload in low Earth orbit, providing a platform of booster stages with which to launch his Martian flotilla.
Von Braun’s plan was, of course, too fantastic in scale to ever be realized, but the kernel of these designs underpinned much of what would follow. The idea that future explorers of Mars would be hurled away from Earth by a brief but violent explosion at the start of their journey and then left to fall freely through space toward their target became the accepted template for human missions to Mars.
Throughout the twentieth century, Mars continued to drift in and out of our thoughts, appearing almost within reach and yet somehow tantalizingly beyond our grasp. Von Braun’s designs envisaged 1965 as the date on which the first humans might arrive at Mars. And since Das Marsprojekt, more than a thousand different technical studies have been conducted, most of them making the assumption that Mars lay little more than twenty years in the future. But that is where Mars has remained: always in our future.
Space is not a single destination. Earth orbit, the Moon, and Mars are as different in character as the continents of the Earth. So too are the voyages and challenges involved in reaching these locations. Low Earth orbit is about negotiating the violence of launch and the terror of reentry, about understanding how we should climb out of the well of gravity in which we live, breaking the bonds of attraction created by the mass of the Earth.
Orbital spaceflight is a furious sprint, with the energies involved barely controlled, an endeavor in which the frailties of human physiology are swamped by the physicality of the propulsive systems. For the pioneers of this age, the ability of the human body to adapt to the extremes of terrestrial environments was largely irrelevant. Dangers were more immediate and dramatic—catastrophic explosions that no one could hope to survive.
Mars presents a challenge of a different scale and character; it’s more a marathon than a sprint. The Moon hangs around a quarter of a million miles away from the surface of the Earth. It is a distance we can easily conceptualize: the number of miles the odometer in your car might clock up before the vehicle seizes and fails. The Moon, the farthest point from the Earth any human in the history of our species has ever traveled, lies close enough to inspect with little more than the naked eye, reachable within four days of spaceflight.
Mars gets no closer than thirty-five million miles away. Its position relative to the Earth is always changing, stretching that separation to as much as four hundred million miles. To cross that gulf, astronaut crews will have to endure missions drawn out over months and years, spanning hundreds of millions of interplanetary miles, traveling thousands of times farther than Armstrong and the Apollo pioneers.
These crews, too, will have to survive the energies of launch and those involved in rocketing them away from Earth and toward Mars. But as they fall across the void that separates the two planets, they will also have to contend with the silent threat of space and its environment. Here the absence of gravitational load takes on a new dimension, transforming from a novelty into a creeping threat.
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THE TERM ZERO-G IS A MISNOMER. Weightlessness in low Earth orbit does not arise because there is no gravity. The gravitational attraction of the Earth doesn’t suddenly melt away to nothing just because we venture 250 miles away from its surface. At that altitude, the force of gravity is only modestly diminished, to around 90 percent of its value at sea level. If you were somehow able to build a house on the end of a pole 250 miles long and live in it, you might have trouble noticing the change. A dropped glass would still break; climbing stairs would still require effort. There might be something of a spring in your step—you and everything around you would be around 10 percent lighter—but you wouldn’t find yourself floating around from room to room. The weightlessness of orbit is experienced not because of the astronauts’ separation from the Earth but because of the way they fall around it.
Weightlessness is something we have all experienced; it’s only that our experience of it is generally so brief as to be barely noticed. If you jump up as hard as you can, you might stay in the air for a little over a second. For that time, you are weightless.
You could prolong the experience simply by falling farther. Imagine standing in a lift on the thirtieth floor of a skyscraper at the moment the supporting cable snaps. From the moment of release until the moment of impact you’d be weightless—a ride of around three hundred feet that would last a little over four seconds.
In the same way, astronauts in low Earth orbit find themselves floating because they are inside a spacecraft that is permanently in free fall around the Earth.
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STRAPPED INTO MY SEAT aboard a modified Airbus, I’m waiting to watch how the French do weightlessness. This is a specialized flight under the auspices of the European Space Agency’s DGA (Direction générale de l’Armement) Essais en vol (literally, “tests in flight”), conducted by pilots who specialize in flying aircraft high into the sky in a parabolic arc and then plunging them into a steep dive, pulling out just in time to avoid disaster. At least, that’s the theory.
There is a flurry of activity before the start of the parabolas. In place of air stewards, we have frequent flyers in tangerine jumpsuits, there to lend a hand if things get rough: the “orange angels.” People get ready, tweak experiments, and position themselves, preparing for the next stomach-lurching maneuver. “One minute,” comes the Gallic voice over the intercom, starting the countdown. The scurrying becomes more frantic.
“Twenty seconds . . . ten seconds . . . pull up!” comes the same disembodied voice. The words are spoken levelly, with no hint of excitement. The person uttering them is at the controls of the plane.
For the next ten to twelve seconds, we are pushed into the foam covering the floor of the aircraft. We experience close to twice the normal gravitational load. The burden of my 168-pound frame is suddenly doubled. I feel as though I’m made of lead.
This is nothing compared to the loads that fighter pilots experience during fast turns, but it’s more than enough to create discomfort. It’s not just toughing out the extra weight; this maneuver is perfect for confusing the hell out of the delicate system of accelerometry in your inner ear.
“Thirty,” calls the pilot in the same level tone of voice, narrating the angle of climb now instead of time. We are on our way up to the top of the roller coaster. That’s exactly how it feels—the nervousness, the anticipation, the excitement—and that’s not far off what it is. Only this ride is 25,000 feet high and will repeat itself thirty times in the next couple of hours.
“Forty,” comes the voice. “Inject.” And then, in one of the most effective rapid weight-loss programs the world has ever known, I go from being 336 pounds to weighing nothing.
They refer to the point at which the plane begins to fall away from you as rapidly as you are falling toward it as injection. It does indeed feel as though you’ve been injected into an alternate reality, one in which the normal laws of physics have been briefly suspended. Around you people and things tumble weightlessly, with no respect for the concepts of up or down. The effects of gravity are suspended here. All those dreams you ever had of flying? Well this aircraft makes them come true for twenty-three seconds at a time.
The Airbus drifts over the top of its parabolic arc, its lift balanced perfectly against its weight, thrust throttled to match drag.
“Thirty . . . twenty . . . pull out.”
After hanging effortlessly in midair one second, I’m smashed back into the deck. The phrase “back down to Earth with a thud” could have been invented for the experience of parabolic flight.
Glancing outside, I see the wingtips flexed two meters out of their normal position, like a tensioned bow. More alarming still, a steady trickle of fuel escapes along the wing edges. Swallowing hard, I turn back to the cabin.
The 1.8-G load pours on. People’s faces appear to age visibly as gravity takes on the skin’s elastin and wins. I’m lying on the deck, still managing a smile, when I catch sight of one of the other passengers, head buried in his arm, sweating beads.
One of the orange angels asks him if he’s OK. He shakes his head vigorously. He’s manhandled to the rear of the plane. There’s a fumble for a sick bag and the familiar sound of retching. It’s not for nothing that they call this the Vomit Comet.
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IN OUR DAILY LIVES, gravity is that pedestrian physical force that keeps us glued to the ground. We don’t think of it as something that shapes our lives. Our bodies are set up to allow us to move within its field of attraction without too much effort, so much so that we barely notice it. You have to go out of your way—climb a cliff face or jump out of a plane—before it starts demanding your attention. But we are constantly sensing the effects of gravity and working against them—largely unconsciously.
We are, for example, equipped with antigravity muscles—those groups that work against the Earth’s force of attraction to keep you standing upright. To get an idea of which groups these are, imagine being on a parade ground with a sergeant major barking at you to stand to attention. Pretty much every muscle you would tense to avoid the prod of his baton is antigravity in function.
Of these the quadriceps, buttocks, and calves, along with a group of muscles—the erector spinae—that surround the spinal column and keep it standing tall, are the most important. Without them the pull of gravity would collapse the human body into a fetal ball and leave it curled close to the floor.
These muscle groups are sculpted by the force of gravity. They are in a state of constant exercise, perpetually loaded and unloaded as we go about our daily lives. It is because of this that the quadriceps, the mass of flesh that constitutes the bulk of your thighs and works to extend and straighten the knee, are the fastest-wasting group in the body.
Your bones, too, are shaped by the force of gravity. We tend to think of our skeleton as pretty inert—there to provide rigidity, little more than a scaffold on which to hang the flesh or a system of biological armor. But at the microscopic level, it is far more dynamic: constantly altering its structure to contend with the gravitational forces it experiences, weaving itself an architecture that best protects the bone from strain.
The biological adaptations to gravity don’t stop there. When you’re standing up, your heart, itself a muscle pump, has to work against gravity, pushing blood vertically in the carotid arteries that lead away from your heart toward your brain.
Even your system of balance and coordination appears to rely in a fundamental way upon the constant force of gravity, with the otoliths—the organs of the inner ear that sense linear acceleration—using it as a sort of calibrating input.
Life on Earth has evolved over the past three and a half billion years in an unchanging gravitational field. In that context, it shouldn’t be a surprise that so much of our physiology appears to be defined by, or dependent upon, gravity. Take gravity away, and our bodies become virtual strangers to us.
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AS A MEDICAL STUDENT, you don’t take the contents of the inner ear very seriously. The organs within detect acceleration and audible stimuli, gathering information about motion and sound. But they are not considered “vital,” in the sense that they are not required to keep the human body alive. As a result, the essential role they play in delivering a finely calibrated sense of motion is often overlooked. However, like all of the best things in life, you don’t really appreciate what you’ve got until you lose it.
The system of accelerometers in your inner ear, the otoliths and semicircular canals, are engineered to provide the finest detail about movement in the ever-changing world about you, creating the illusion that you are essentially a stable platform through which the world can be observed as though it were a film made with a Steadicam. It is a system that shares its inputs and outputs with the eyes, the heart, the joints, and the muscles.
Consider for a moment the act of looking at stuff. Hold a finger up in front of your eyes. Now shake your head left and right as though you are vigorously saying no. The image of your finger remains remarkably stable, doesn’t it? Now try keeping your head still and waggling your finger back and forth at the same rate. This time the image is less stable; plenty of blur creeps in.
Keeping an image stable and clear in your visual field is a pretty difficult task to achieve. First you have to focus the image onto the layer of light-sensitive cells at the back of your eye called the retina. Now, your retina isn’t the same all over. At the rear, near the center, is a cluster of densely packed cells, conelike in shape, that account for less than 1 percent of the area of the retina. This tiny but all-important area is called the fovea and is responsible for tasks such as reading or studying a picture. This high density of specialized cells resolves the critical detail of a scene and its colors. The rest of the retina, by comparison, is populated by rods—good in low-light conditions but rubbish at subtlety. Uninterested in nuance, they’re there chiefly to pick up movement in the periphery, to identify a target on which you should focus your attention more closely.
Those receptors report to a specialized part of the brain called the visual cortex. What’s interesting is that, although the fovea accounts for less than a hundredth of the surface area of the retina—one voice among a hundred—the visual cortex dedicates 50 percent of its mass to listening to the superdiscriminating fovea.
All this effort, and we’re still talking about a stationary eyeball focusing on a stationary object.
Now let’s start shaking things up a bit. Imagine that the thing you’re looking at is no longer stationary but is instead moving. As it moves, you have to rotate your eyeballs to keep its image focused in the right spot. Once it reaches the point at which you can’t track it with your eyes anymore, you start to move your head, too.
Now you have two spheres, capable of rotating independently, carrying a lens system that is trying to keep the image of a moving object sharp, on an area at the back of the eye that is only a few millimeters across.
It is the slaving together of the accelerometers in your inner ear, the muscles that rotate your eyeball, and those that turn and tilt your head that allows you to achieve this remarkable feat.
Now imagine that the system doesn’t work and that the stable image of the world you take for granted is replaced by a gently oscillating, nausea-inducing scene from which there is no escape. If you’ve ever suffered from seasickness, imagine the worst possible episode of that, on a ship that you are never allowed to leave and under which the rolling seas will never calm down. That’s what it feels like when the organs of the inner ear malfunction. And that can be caused by disease, drugs, poisons, and—as it turns out—the absence of gravity.
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WEIGHTLESSNESS MAY SOUND LIKE FUN, but the majority of rookie astronauts feel sick in the first forty-eight hours of spaceflight. Antiemetic medications—those drugs that act to combat feelings of nausea—are among the most commonly prescribed during NASA spaceflights. The undesirable effects don’t stop there.
Deprived of gravitational load, bones fall prey to a kind of space-flight-induced osteoporosis. The balance between the populations of cells responsible for laying down bone and those for removing bone is lost, and so bones become less dense and more prone to fracture. And because 99 percent of your body’s calcium is stored in the skeleton, as it wastes away, that calcium finds its way into the bloodstream, causing yet more problems.
Hypercalcemia—a pathological state in which the levels of calcium in the blood are raised—is famous for causing a tetrad of clinical problems. Constipation is the least of these, followed by pains in the long bones. More seriously, renal stones can form, blocking the route from your kidneys to your bladder, causing excruciating pain. And finally there is the possibility of psychotic depression. Medical students remember this list as: bones, stones, abdominal groans, and psychic moans. All four are problematic when you could be two years and more than four hundred million miles from your closest family practitioner.
It’s not just your bones that waste away. Muscles do too—the antigravity groups at an alarming rate. In experiments that charted the changes in the quadriceps of rats flown in space, more than a third of the total muscle bulk was lost within nine days. More interesting still, astronauts’ muscle fiber switches from slow twitch—the efficient, fatigue-resistant type suited to marathon running—toward the fast-twitch variety that a sprinter might prefer.
Meanwhile, the heart and its system of vessels, deprived of the need to work against the force of gravity, become deconditioned. Spaceflight enforces a sedentary existence on otherwise well-exercised physiological systems, slowly taking athletes and turning them into couch potatoes.
For the cardiovascular system, the finely tuned reflexes that on Earth constantly cope with changes in posture sharply deteriorate during extended spaceflight. Picture yourself lying on the sofa, watching back-to-back movies. The doorbell rings, and you spring to your feet; your cardiovascular system is forced to make a sudden alteration. Having gone from lying to standing, the blood in your body now suddenly tries to pool in your lower limbs, reducing the volume that returns to the heart and as a consequence the force with which it beats. In addition, the blood that was lazily flowing between your heart and brain along your carotid arteries is now trying to travel vertically against the pull of gravity.
Combined and unopposed, these changes will leave your brain deprived of an adequate blood supply and you unconscious on the floor.
All that stands between you and that fate is a reflex that senses the drop in pressure in the carotid arteries and tells the brain to increase the rate and force of contraction of the heart, while simultaneously constricting peripheral blood vessels to restore blood pressure. This primitive reflex is all-important. Without it, you’d end up lying in a crumpled heap every time you stood up too suddenly.
This is what we see in astronauts returning from long missions aboard the space station. Asked to stand still and upright for ten minutes, a significant fraction are unable to do so without feeling faint. This we call postflight orthostatic intolerance—an inability to maintain an upright posture.
The impairments don’t stop there. There are other, less well-understood alterations. Red blood cell counts fall, inducing a sort of space anemia. Immunity suffers, wound healing slows, and sleep is chronically disturbed.
In short, most astronauts return from long-duration spaceflight—missions of more than six months—in a temporarily diminished state: sleep deprived, their cardiovascular system deconditioned, their muscles and bones weakened, and their hand-eye coordination impaired. As blissful as the experience of floating around might appear, it erodes the body’s ability to function when challenged again by the force of gravity.
When astronaut crews arrive back on Earth, they are met by a support team that includes nurses and physicians, and they are spirited away to recuperate from the experience. And even then, with all the care that the assembled terrestrial recovery forces can muster, there are still incidents. Returning crew members have been known to vomit at celebratory banquets, collapse in showers, or run their vehicles off the road because of transient disorientation.
Others, forgetting that they have returned to a world ruled by gravity, drop expensive equipment or fragile gifts, having got used to the idea that released objects float rather than sink to the floor. Back at home, one astronaut reportedly got out of bed to change his infant son’s diaper and stood for a while wondering how he might Velcro the baby to the cot while he searched for some wipes.
The problems of spaceflight are principally those of readaptation to a world in which gravity is the shaping force. Reacclimatizing to that, both physically and psychologically, is a challenge. On return to Earth, astronauts are carefully monitored while their bodies readapt. But on a mission to Mars, they’d arrive and be entirely on their own.
The crews that arrive at Mars would do so after six to nine months of flight and would experience many if not all of these problems. There they would have to perform the most challenging landing in the history of human spaceflight. The communication delay between Earth and Mars might be up to twenty minutes. In that moment of touchdown, they would be truly alone. Assuming they land safely—and remember that around 50 percent of everything we’ve thrown at Mars has crashed or disappeared—they’d then have to leave their vehicle to walk to the pre-prepared habitat. That habitat might be up to half a kilometer away.
And that’s assuming they even make it that far.
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IT’S WORTH BRIEFLY CONSIDERING what it takes to get to Mars. The term spaceflight is something of a misnomer. Human-rated spacecraft don’t really fly through space. Their rocket motors fire for only a few brief minutes at the start of the journey, throwing the vehicle and its occupants toward their intended target, like a medieval ballista hurling a missile at the walls of a castle. The spacecraft have their own rocket motors and thrusters, but these are far less powerful than the launcher that set them on their way. Once they’re traveling, only subtle course corrections can be made. So astronauts on their way to their destination are engaged in an activity that might more accurately be described as spacefall.
While the vehicle and its crew are busy falling across space, Mars is out there somewhere in the darkness, tearing around its elliptical orbit at a little over fifty thousand miles per hour. Mars’s journey around the Sun takes 687 days. Earth completes its orbit in 365.25 days, moving at around seventy thousand miles per hour, which leaves the two planets constantly changing their relative positions in the sky.
This has consequences. It means that you can’t decide to go to Mars any time you want. You have to wait for precisely the right opportunity, launching from low Earth orbit at exactly the right time, so that Mars is there when you arrive. And the same is true upon your return.
Despite these restrictions, there are as many different recipes for getting to Mars as there are for the perfect chicken noodle soup. Mission architects have to juggle propulsion systems, trajectories, vehicle velocities, and atmospheric entry strategies and trade these against payload mass and crew size in an attempt to design something realistic in terms of risk and cost. They have to decide, for example, between exotic deep-space maneuvers—which might use the orbital energy of Venus as a slingshot to propel vehicles on their way to and from Mars—and more prosaic but potentially safer journeys.
But in the end, all of the mission designs boil down to two broad scenarios: those that see you arrive and stay on Mars for a few weeks and those that leave you on the surface of the Red Planet for more than a year. These are the so-called short-stay and long-stay mission architectures for Mars.
For the short-stay missions, crews would travel for close to nine months to get to Mars. But once there they could then take advantage of an early opportunity to return to Earth, which would arise between thirty and ninety days after their arrival. This, after having spent close to nine months in flight, would be like flying from London to New York, milling around in the gift shop at JFK for an hour, and then flying straight home. But it has the advantage of shortening the total mission duration to less than twenty-four months.
For the long-stay missions, you can get to Mars a little faster, closer to six months than nine, but in this case, the elliptical movements of the planets mean you don’t get a chance to come home again for something like eighteen months.
That means you’d spend at least a year traveling and a year and a half or more on Mars. That mission would approach three years in duration—all of which would be spent weightless or working in the reduced gravity of Mars.
There are a number of formidable problems that accompany missions of such duration. The first is life support. How do you invent a system that can keep a crew of four alive for nearly three years? For space stations, breathable oxygen is generated by electrolyzing water: using a current to decompose it into hydrogen and oxygen. This requires a steady supply of water, which is conveniently resupplied from Earth via the Russian Progress vehicles: automatically piloted, space-age delivery trucks. The carbon dioxide that would otherwise accumulate is scrubbed out using chemical sieves—canisters of lithium hydroxide that react with the CO2 and remove it from the atmosphere. These too need to be resupplied aboard the Progress vehicles, along with food for the crew.
But there is no easy way to resupply a team traveling to Mars, and so a number of ingenious solutions to this problem have been proposed. One involves a grow-your-own approach to life support and nutrition.
One of the experiments under way when I first visited Johnson Space Center in 1997 was exactly this. Plants respire photosynthetically, by taking in carbon dioxide and generating oxygen and water. It turns out that if you grow ten thousand wheat plants, you can generate more than enough oxygen to breathe while removing the human waste gas of carbon dioxide. Better still, you have a partial source of nutrition. For a while, the Space Center had a team of four volunteers locked up in a hermetically sealed tube, subsisting pretty independently on this self-regenerating, hydroponically grown life-support system. And that’s all great—until you factor in the possibility of crop failure.
Another solution, discussed at a European Space Agency human space-exploration symposium, would be to grow vats of algae, which might be easier to sustain than wheat and would also provide a source of protein. Between that and the wheat plants, you could get halfway to a diet of pizzalike food—bread coated with flavored algae—and massively reduce the weight and volume of the food and life-support apparatus required for a Mars mission.
After that conference, I remember listening wide-eyed in the bar while an excitable Frenchman who specialized in the field of regenerative life support told me how it might work, going so far as to explain the recycling of urine and the use of feces as a source of fertilization.
“You see,” he shouted above the din of the bar, “these people who go to Mars, they will literally ’av to eat their own shit.”
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IF THAT HASN’T PUT YOU OFF the trip already, then consider the radiation hazards. As far as anyone can tell, the background radiation you would be exposed to while traveling between Earth and Mars should be within safe limits—unless there’s a solar flare.
These giant eruptions of plasma from the surface of the Sun are accompanied by an intense shower of high-energy particles that rain through space. For the astronauts and cosmonauts operating in low Earth orbit, within the cage of protection provided by the Earth’s magnetic field, this presents little problem. The charged particles are caught and trapped by the lines of Earth’s magnetic flux, depositing their energy more or less harmlessly, well away from the human crews.
But for a vehicle venturing outside the Earth’s immediate neighborhood, there is no such protection. A solar flare is like a neutron bomb going off next to you. Energetic particles—charged helium nuclei, neutrons, protons, and the like—would pass through your body, wreaking havoc and irreversibly damaging cells. Such an exposure would be like taking the DNA blueprints of each cell, shooting cannon balls through them, and then trying to build something based on the information that remains. The resulting structures would be dangerously unstable and prone to malfunction.
The fastest-proliferating cell populations would be worst affected: hair follicles, skin, and the lining of the gut. The rapidly dividing cells of the bone marrow, too, would fall victim. With blood cells decimated, the sufferer would be left anemic, short of platelets to help clot blood and bolster the immune system. This explains the familiar depiction of acute radiation sickness: hair falling out in clumps, diarrhea, bruised skin, and bleeding gums. Without a shield, it would be impossible to survive such an exposure.
To make matters worse, solar flares arise sporadically, and we’re about as good at predicting them as we are at forecasting the British weather. And there’s no straightforward way of combating their effects. Building a ship coated with lead wouldn’t help—even if you could find a way to lift that mass into orbit. Lead and other heavy metals are great at shielding against X-ray radiation and lighter particles, but when it comes to highly energetic heavy particles, they are worse than useless. Massive particles, arriving at close to the speed of light, would smash into the atoms of a metal shield and scatter them like a cue ball hitting a billiard pack. These scattered atoms would then give rise to secondary radiation, as deadly as the particles they were supposed to shield against.
One possibility lies in building a sort of bomb shelter in the spacecraft, an area more resistant to the radiation storms brought by a solar flare. This you could shield, not with layers of metal, but with a jacket of water. It turns out that water is very good at attenuating solar particle radiation. But this is pretty speculative. When it comes to the radiation hazards of a human mission to Mars, if you ask the experts, they tell you that we simply don’t yet know enough.
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EVEN IF WE FIGURE OUT A way to negotiate the radiation and build a life-support system that is at least partly regenerative, we keep getting back to the most elemental problem: having to contend with the absence of gravity. The longest mission in human spaceflight history was 437 days, 17 hours, 58 minutes, and 16 seconds; it was completed by cosmonaut Valeri Polyakov aboard the Russian space station Mir between 1994 and 1995. By all accounts, he arrived back on Earth in reasonably good health, but it is far from clear that this would be true of all space explorers.
Polyakov is in an exclusive club. Around five hundred people have flown into space. Of these, only ten have flown for more than two hundred days and only two for more than a year.
Most of our experience in the field of astronautics involves missions of less than two weeks’ duration. The impairments seen in crew members who have flown for between three and six months are significant and tend to vary from individual to individual.
A range of countermeasures to combat the effects of longer missions is available to astronaut crews. These include medications, special diets, and regimens of resistive exercise. And while they have gone some way to mitigating the consequences of human spaceflight, none appears uniformly effective.
It is because of this that the idea of generating artificial gravity has surfaced time and time again. The concept is not new. The earliest rocket scientists realized that their crews would experience weightlessness and that this might be problematic, even if they could not predict all of its effects.
In 1923 Hermann Oberth proposed a solution: a vehicle tethered to a counterweight that would spin end over end like a twirling baton, subjecting the occupants to an artificial gravitational load as it went. It’s the same load we feel on spinning fairground rides, the force that pins us against the side of the car.
So far, so good. But the problem with artificial gravity lies not in the underlying physics of the idea but with engineering a rotating vehicle capable of the feat. Here design is narrowly constrained by the biological frailties of the astronaut crew.
The force of artificial gravity generated by a rotating vehicle depends upon the radius of the vehicle and its rotation rate. To generate enough force it must either be small and spin extremely quickly or be large and spin more slowly.
Everybody differs in their tolerance to fairground rides; some people can be spun at head-snapping rates without apparent ill effect while others feel sick just watching the thing go around. This, again, is down to the apparatus of the inner ear: detecting rotational accelerations, trying to make sense of what is happening, and expressing displeasure through the vomiting center if it cannot. But if the rate of rotation is kept slow enough, to four revolutions a minute or less, everybody in time can adapt to the motion.
With that requirement fixed, the radius of rotation necessary to produce a force of 1 G—equivalent to the load you would feel at the surface of the Earth—can be calculated. It leaves you with a vehicle around 125 meters across—coincidentally about the same size as the London Eye. If the thought of something of that size whacking around four times every minute seems daunting, imagine building a vehicle of that scale and then launching it into space.
NASA did more than imagine. In the 1990s, Kent Joosten and a team of engineers at Johnson Space Center came up with a broad-brush design for an artificial-gravity vehicle that might actually work. This returned to Hermann Oberth’s original idea of a tether between a crew habitat and a counterweight. In Joosten’s design, the module and its counterweight were separated by an ingenious, ultralight, liquid-crystal pylon structure. This could be compressed and stored during launch from Earth and then deployed after the vehicle had arrived in orbit. The whole thing would then tumble end over end all the way to Mars, with the crew living in a module about the size of a four-bedroom house under conditions that approximate terrestrial gravity.
Joosten’s artificial-gravity study represents the most mature technical approach to the subject so far seen. There are, however, a number of significant problems to be overcome before such a vehicle design can be realized. It presents an entirely new paradigm in our concept of what human spaceflight is, and this has in part contributed to a reluctance to embrace or further investigate the idea.
Among the hundreds of studies that have considered how best to get to Mars, nearly all of them have involved smaller, simpler vehicles of the type that took us to the Moon. But there is a way to deliver artificial gravity inside such spacecraft, even if the vehicle itself can’t be spun.
In our daily lives, our bodies do not experience constant gravitational load. When we stomp up and down stairs, our joints become shock-loaded, with regions of our skeleton transiently experiencing up to three or four times the gravity they would at rest. When we lie down to sleep, the long axis of our body is more or less perpendicular to the force of gravity, and our skeleton, cardiovascular system, and antigravity muscles are left unloaded. This quasi-weightless state quite closely resembles the weightlessness of spaceflight. Indeed, when researchers want to mimic the effects of microgravity here on Earth, they simply send a bunch of people to bed.
So on Earth our physiology is maintained by only intermittent exposure to gravitational load—the standing up and stomping around we do during the day. And even that isn’t constant. From this realization grew the idea that we might prescribe gravity like a drug, giving it in short but large doses. Cue the short-arm centrifuge as a countermeasure to the effects of weightlessness. Instead of building a spacecraft as big as the London Eye and rotating it slowly, you could build a much smaller spinning device, rotate it very quickly, and pack that inside a conventional spacecraft module.
If you do the math on this, a centrifuge with a radius of three meters would have to spin around forty times a minute to generate a load of about 3 G at its edges. This bizarre regimen of loading might nevertheless be enough to protect the body from weightlessness. Better still, it can be administered in short doses; as little as an hour a day might be sufficient. And with this knowledge in hand, NASA went out and built one.
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SOMEWHERE IN A NASA LABORATORY in Galveston, the ceiling spins around above my head, revolving forty times a minute. I keep my head straight, eyes fixed on the screen mounted above, about three feet from my face.
Deep within my inner ear are tiny cells with hairlike protrusions that waft in a gel like blades of grass standing vertically, set in a plate of jelly. Part of my vestibular system, these exist to detect acceleration in the world around me. The more the jelly leans over, the more the blades of grass bend, and this triggers the firing of the hair cells. Right now they’re struggling to make sense of what I’m being put through.
The set of hair cells in my semicircular canals, the organs that detect rotation, are screaming, firing constantly with the whirling of my body. My brain got bored of listening to that quite some time ago and has decided to ignore their messages, leaving me feeling almost comfortable. But it’s a precarious state. There is profound conflict between what I’m seeing and what I’m feeling. My vomiting center—which is wired in to the same box of tricks that senses acceleration—is at this instant just about managing to stay quiet. I have to keep my head dead center to maintain that status quo. If I start jerking it around, I’ll be vomiting in seconds.
I’m wearing a headset with a microphone. A researcher in the control room, watching the camera feed, asks me if I’m still OK. I tell him that I am. Another voice from the control room bombards me with a few more questions and then asks me if I wouldn’t mind turning my head to take a look at a piece of equipment he’s worried about on my right-hand side. I tell him that I’m not falling for that one. Somewhere off-mike, there’s an evil chuckle.
I’ve been here now for half an hour; there are still another thirty minutes left. I am lying on my back on this experimental device: a centrifuge small enough to be accommodated in the module of a spacecraft on its way to Mars.
It looks, at first glance, like an instrument of torture. A pair of arms, each one just about long and wide enough to accommodate an adult lying supine, sprout from a central column. There are harnesses and straps to stop you from flailing around and probes and monitors designed to extract information from you. The whole thing can rotate at a stomach-wrenching rate. If Tomás de Torquemada invented a fairground ride, it would look something like this.
The apparatus is there to interrogate human physiology, to determine how it will respond to this insult. ECG electrodes are glued to my chest, an automated blood-pressure cuff inflates periodically, and a probe monitors the oxygen in my bloodstream.
This is a device for generating artificial gravity—or at least an artificial gravitational load. The forces generated when the machine rotates force me out, trying to fling me toward the walls of the room. I’m stopped from doing so by a plate at my feet. As the centrifuge spins up, I get heavier against that plate. At full tilt, the force on my body below my waist is between two and three times that of normal gravity. In my upper body, where the speed of travel is slower, the load is less. It means there’s a gradient of force along my body that builds steadily from head to toe. This gives the illusion that I’m lying with my back arched, making me feel as though I’m engaged in some sort of limbo dance maneuver.
As I settle down into it, I begin to feel more comfortable, comfortable enough to begin to get bored. If I don’t move my head around, the whole experience is quite doable, almost relaxing. A voice crackles into my headset.
“How’re you doing?” asks the researcher. I tell him I’m fine. “We can stick something on the screen if you’re getting bored.” He fumbles around in the control room and slides a DVD into the player. A Harry Potter film springs into view on the screen above me, and all of a sudden, whirling in the darkness in front of a small glowing screen feels no more abnormal than watching an in-flight movie on a long-haul flight. And I begin to think that this could be an OK way to get to Mars after all.
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ARTIFICIAL GRAVITY IS ONE OF THOSE things that people tend to dismiss with a snort if they don’t know much about it. It remains unclear how long humans can be deployed in space without suffering serious medical consequence, but it is unlikely that we can endure weightlessness indefinitely and maintain acceptable health. If we are to continue to push out into space, then at some point, artificial-gravity machines—compact torture chambers or giant twirling batons—will have to play a role. This is a natural progression. We take everything else with us into space: our light, our heat, our food and water; we even take our atmosphere. At some point, it seems certain that we’ll take gravity with us too.
There’s a lot of work to be done before that can happen. We are unsure of the prescription, of how hard and how fast you would need to spin crew members to protect them from the ravages of weightlessness. Neither do we know how much protection the lesser gravity of Mars will provide, if any.
It is unclear what such a system might do to the inner ear. Early results from NASA’s Artificial Gravity Pilot Project suggested that the heart and muscles might be usefully protected in this way. It would be surprising if bone didn’t benefit too. But the inner ear and its organs of accelerometry are a different story. This strange rotational input might, over time, lead to maladaptive changes that might worsen their function. On the other hand, it might prove highly protective. Sadly, it doesn’t seem that we’ll find out the answers anytime soon.
In 2009, just as the artificial-gravity project was ready to enter a more comprehensive phase of investigation, a series of budget cuts tore through NASA. The strategy that would have seen the short-arm centrifuge investigated thoroughly on the ground and then made ready for flight aboard the space station was canned. It isn’t the last we’ve seen of this; as one of the investigators quipped, “Artificial gravity is an idea that comes around and around. . . .”
About the same time, a new vision was set, one that prioritized a return to the Moon over a first human mission to Mars. And once again the Red Planet receded into the future.